1. Field of the Invention
The present invention relates to a negative pressure type booster device for motor vehicles and particularly, to a negative pressure type booster device capable of compensating for the insufficient stepping force exerted on a brake pedal at the time of emergency braking.
2. Discussion of the Related Art
Heretofore, the result of a certain analysis has taught that at the time of emergency braking wherein the driver quickly steps a brake pedal in the event of the sudden emergence of an obstacle or block on the course of the vehicle, it is often the case that the braking capability of the vehicle cannot be sufficiently performed because the stepping force the driver exerts on the brake pedal is considerably smaller than that needed to generate the fluid pressure which brings road wheels in lock states. On the basis of the result of such an analysis, there has been developed negative pressure type booster devices which are capable of judging whether the braking applied is an ordinary one or an emergency one from the stepped speed of the brake pedal or the speed in the increase of the fluid pressure of a master cylinder and at the time of the emergency braking, of automatically increasing the brake fluid pressure to the extent that the road wheels are brought into lock. In negative pressure type booster devices capable of assisting the stepping force on a brake pedal at such an emergency braking, for example, in one described in European patent application No. EP 1 227 020 A1, the interior of a booster shell is divided by a partition member into a variable pressure chamber and a constant pressure (i.e., vacuum) chamber, and an output piston of a bottomed, cylindrical shape is secured at its base portion to the partition member. The movement in the forward and backward direction of the partition member caused by the pressure difference between the both chambers is transmitted from the output piston to an output rod through a reaction mechanism. A plunger operated in connection with the reaction mechanism is connected to an input rod axially movable by a brake pedal. The output piston is provided with a small-diameter hole and a large-diameter hole respectively at a base end side and an open end side, and a negative pressure valve seat is annularly protruded along the internal surface at a stepped portion between the small and large-diameter holes. Passages communicating with the constant pressure chamber open at a shoulder surface of the stepped portion diametrically outside of the negative pressure valve seat, and an atmosphere valve seat is annularly formed at the rear end surface of the plunger at a position behind the negative pressure valve seat to encircle an air leading passage. A cylindrical valve member which is constituted by joining forward and rear walls with an outer circumferential wall thereby to take a U-shape in the half cross-section is received in the output piston movably in the forward and backward direction. The cylindrical valve member is provided at front end surfaces of the forward and rear walls with a negative pressure valve and an atmosphere valve, which are contactable respectively with the negative pressure valve seat and the atmosphere valve seat for bringing the variable pressure chamber selectively into communication with either of the constant pressure chamber and the atmosphere or isolation therefrom. The cylindrical valve member is urged forward by means of a compression spring.
Further, a cylindrical valve seat member encircling the plunger is air-tightly inserted into the small-diameter hole of the output piston to be slidable in the axial direction, and a second negative pressure valve seat is formed at the rear end of the valve seat member radially inside of the negative pressure valve seat. A second negative pressure valve contactable with the second negative pressure valve seat is provided on the forward end surface of the cylindrical valve member, and passages are provided on the lateral sides of the output piston and the valve seat member for making the inside of the valve seat member communicate with the variable pressure chamber. The valve seat member is urged backward by means of a spring member so that the second negative pressure valve seat comes closer to the cylindrical valve member than the negative pressure valve seat do. Latch means is provided on the base portion of the output piston for latching the valve seat member at an ordinary position against the resilient force of the spring member so that the second negative pressure valve seat is separated forward from the cylindrical valve member.
In the known device of the aforementioned construction, when the brake pedal is quickly stepped on to advance the plunger beyond a predetermined distance relative to the output piston, the latch means releases the valve seat member, whereby the second negative pressure valve seat is brought into contact with the second negative pressure valve formed on the front end wall of the cylindrical valve member thereby to move the same backward. This causes the atmosphere valve to be separated from the atmosphere valve seat, so that the variable pressure chamber is brought quickly and compulsorily into communication with the atmosphere. As a consequence, a larger propelling force than that in the ordinary state is output to the output member, so that it can be realized to generate a sufficiently high fluid pressure in the master cylinder. At the moment that the brake pedal is stepped on, the ratio of the output to the input becomes the infinity until the plunger comes into contact with the reaction member, and the resultant output characteristic indicates a jumping characteristic wherein the output increases on a step-by-step basis. At the time of emergency braking, the latch means releases the valve seat member and permits the second negative pressure valve seat to come into contact with the second negative pressure valve thereby to push the cylindrical valve member backward. This causes the jumping characteristic to become more notable than that at the time of the ordinary braking, whereby a larger propelling force than that at the time of the ordinary braking is output to the output member. That is, the latching and releasing of the valve seat member by the latch means makes it possible to selectively switch the two kinds of the input-output characteristics for the ordinary braking as well as for the emergency braking. When the brake pedal is released, the plunger is brought into the state that it does not advance more than the predetermined distance relative to the output piston. This causes the valve seat member to advance relative to the output piston, whereby the valve seat member is latched by the latch means to the ordinary position.
When the latch means releases the valve seat member at the time of the emergency braking, the valve seat member is moved backward by means of the spring member, and therefore, the cylindrical valve member has to be moved backward quickly through the contact of the second negative pressure valve seat with the second negative pressure valve. For this purpose, the cylindrical valve member is to be downsized and lightened, and the resistance against the movement of the cylindrical valve member is to be diminished. However, the known device of the aforementioned construction is advantageous in that the cylindrical valve member of a U-shape in the half cross-section is constructed to have a smaller resistance against the movement thereof by being provided with the negative pressure valve and the atmosphere valve respectively annually on the forward end surfaces of the front and rear walls thereof, but has drawbacks in that the cylindrical valve member becomes complicated and elongated in the axial direction to be enlarged as well as to be increased in mass.
Further, in order that the pressure difference between the constant and variable pressure chambers does not cause any propelling force to be exerted on the cylindrical valve member, the area through which the pressure difference acts on the cylindrical valve member has to be small. However, in the prior art device, because the negative pressure valve seat is provided annually to encircle the plunger, the circumferential length of the negative pressure valve seat and the area at which the pressure difference between the constant and variable pressure chambers acts on the cylindrical valve member cannot be set or designed independently of the diameter of the atmosphere valve seat, and it may be the case on occasion that the air discharge from the variable pressure chamber to the constant pressure chamber is not highly responsive and silent.
In addition, the prior art device of the aforementioned construction is complicated in the shapes of those components which constitute a mechanism for increasing the output at the time of the emergency braking, and therefore, is low in productivity.